Tubing and duct systems for conveying fluids are in widespread use in many industries. In the aerospace industry, welded ducts are used in the environmental control system and in the wing de-icing system for conveying heated air from the engine to the leading edges and nacelle inlet nose to prevent ice from forming on those critical surfaces in icing conditions in flight. These and other duct systems have elbows, “T” ducts, flanges and other components used to assemble the complete system. A “T-duct” is a short length of tubing having an integral tubular protrusion from the duct side wall by which a side duct can be attached, as by welding or coupling hardware, into a duct line. This protrusion is commonly known as a “pullout.”
Two methods for making a tubular part, such as a “T” duct, with an integral pullout are taught in U.S. Pat. No. 5,649,439 issued on Jul. 22, 1997, to David W. Schulz entitled “Tool for Sealing Superplastic Tube.” Both methods use gas pressure to superplastically form a portion of a side wall of an end-sealed tube, heated to superplastic temperature in a die, into a side pocket of the die to form the pullout. The formed tube is cooled and removed from the die, and the end of the pullout is trimmed off to remove the cap and to give the pullout a planar lip.
These methods reliably and repeatably produce parts as designed, but have one shortcoming that, in aerospace applications in particular, has significant economic consequences. Since the end cap of the pullout bulge must remain intact to contain the pressurized forming gas, the material in the cap is not available for use in the pullout side wall. Accordingly, to prevent excessive thinning of the pullout, a thicker tube than is required by the engineering specifications for that duct system must be used. That thicker tube, carried just to avoid the excessive thinout of the pullout lip, can add several pounds to an airplane de-icing duct system, for example. In the aerospace industry, in particular, wherein weight is an important factor in the design of any system, even a few pounds of weight in excess of that required by the engineering specifications is looked upon with disfavor.
Another problem with excessive thinning of the pullout on a tubular part occurs when the mating duct is welded to the pullout. Welding of thin-wall ducts and tubing requires careful control of the welding power and speed to obtain a weld bead with the desired penetration and mass, and to avoid burn-through or other over heating problems. Welding a pullout joint that has been thinned, to a fresh section of straight tubing with a thicker wall, presents a difficult challenge that requires the skills of a master welder. Oftentimes even the best welders are unable to manage keeping an even weld bead or avoid blow-through holes because of the difference in the amount of parent material being melted around the pullout. Many parts are scrapped because of non-conforming weld bead width, insufficient weld penetration, blow holes, weld-line porosity, inclusions and other defects that can be attributed to the variation of thickness surrounding the pullout.
The radius area where the pullout joins the tube is a high stress area on an airplane de-icing duct system due to bending stresses caused by movement of the wings in flight, thermal stresses and sonic fatigue. All of these factors generate stresses that are transmitted along the spurs of the duct to the joint at the formed pullout radius where the pullout meets the mainline section of the straight tube. For this reason, there is a structural benefit in locating the weld bead of the tube welded to the pullout as far as possible from the pullout radius, so the stresses that are concentrated at the pullout radius are not concentrated at the weld bead, since the welding process introduces defects such as porosity in the weld and decreases the structural load capacity of the duct around the weld.
Another existing tube pullout production technique is a ball pulling process that is used to produce the same type of aerospace ducting tee's and joints. A round hole is cut in the sidewall of a tube in a position where the pullout is to be formed. A ball that is slightly larger in diameter than the hole is pulled through the hole to form a pullout with the same inside diameter as the outside diameter of the ball. The process is designed in such a way that the ram of a hydraulic actuator can be run up inside the tube through the hole, a ball screwed onto the threaded end of the ram, and the ball pulled through the hole using the hydraulic action of the actuator. The pullout shape is controlled by a die which has a machine cut draw radius around which the pullout forms as the ball stretches the material outward.
An enhanced ball pulling process heats the ball to a temperature of about 1000° F. During pulling, heat from the hot ball is conducted to the tubing material in the region that will be stretched into the pullout, heating it to an elevated temperature, near the temperature of the ball. A slight increase in ductility is realized by heating the ducting material. For example, the possible elongation of commercially pure titanium made in accordance with Mil Standard Mil-T-9046J, CP-1 at room temperature is about 25%; at 1000° F. its possible elongation is about 28%.
The problem with the conventional or heated ball pullout process is cracking and excessive thinout around the lip of the pullout. The forming stresses and elongations that result during forming often surpass the formability limits of the material. The strain needed to form the pullout causes a high scrap rate due to cracking. Aerospace ducting systems are usually designed to approach the minimum thickness to save weight, hence thinout at the lip of the pullout can reduce the lip thickness below the acceptable minimum. Many parts are scrapped because the pullout lip is thinner than this engineering designed minimum thickness.
The conventional pullout forming process has many variables that contribute to the high scrap rate. The ductility of alloys used in ducting systems can vary from lot to lot. Elongation differences of only 1 or 2% in the raw material properties can have a significant impact on cracking and thinout.
In addition to variations in the material, it is difficult to precisely locate the hole cut in the tube relative to the position and linear path that the ball travels when the pullout is made. A misalignment of even 0.005″ can have a significant effect on the elongation of the pullout sidewalls. Many process failures occur in which the pullout depth is slightly short on one side and is longer and cracked on the opposite side, resulting from slight misalignment of the hole with the ball travel path.
Because the conventional pullout forming process causes thinout in the same location that is the most highly stressed, welded duct systems in airplanes have always been designed with thicker tube walls than would otherwise be necessary, thereby increasing the weight of the airplane duct system. The weight is especially undesirable in wing de-icing systems because there is a multiplier effect for the impact of weight for weight added to the wings.
Thus, there has long been an unsatisfied need in the industry for a process for making pullouts that does not suffer from excessive thinning of the rim of the pullout and which avoids cracking or bursting in the highly strained regions around the rim on the pullout. The benefits of producing a flange, pullout, or T-duct with reduced thickness variation would extend to both aerospace manufacturing and design capabilities, and also to commercial and industrial applications.